Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones etc., have evolved during the last decade into systems that must utilize the radio spectrum in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bit rate. In order to realize such systems, much effort has been spent in developing both software and hardware components that can provide these capabilities. One vital hardware component that is needed in all radio communicating arrangements is an antenna, and in many implementation scenarios a compact and simple antenna construction is desirable.
Substrate integrated waveguides, SIW, realize planar rectangular waveguides in printed technology by using a thin substrate material covered by metal plates on its top and bottom surfaces. Via holes or via posts on each side act as walls to create the waveguide. When the diameters of the vias and their period are chosen properly as to emulate a perfect metallic wall, the well-known dispersion relations of the rectangular waveguide will become a good approximation for the dispersion performance of the SIW. It is well-known how to design the vias in SIWs. Its compact and low profile characteristics are very attractive toward the current tendency to use the mm-wave radio spectrum for wireless communication and to integrate radio frequency, RF, components and antennas in compact and small modules.
A SIW antenna can be realized using printed circuit board, PCB, technology. In particular, the so-called SIW horn antenna has been introduced for application requiring end-fire radiation, i.e., antennas whose maximum radiation is in the direction parallel to their axis. Such a prior art SIW horn antenna 100 is schematically illustrated in a perspective view in FIG. 1. The SIW horn antenna 100 comprises a substrate 102, e.g. part of a PCB, covered on a top surface by a first metal plate 104 and on a bottom surface by a bottom metal plate 106. Vias 108 connect the top and bottom plates 104, 106 and the vias 108 are arranged in a flared configuration in the x/y directions from an input end 110 and terminate in an aperture 112 as FIG. 1 illustrates with reference to an xyz coordinate system.
By extending the substrate 102 beyond the antenna aperture 112 in the direction of extension, y, as shown in FIG. 1, a narrower beam width can be reached in the E-plane (i.e. in the z-direction indicated in FIG. 1), thus increasing the overall antenna directivity. However, the SIW horn performance, such as bandwidth and front-to-back ratio, FTBR, is strongly affected when the substrate thickness is much smaller than the wavelength mainly due to the mismatch at the dielectric-air interface, which usually is the case for SIW structures. This effect is clear at frequencies below 20 GHz, where commercial substrates have substrate thicknesses on the order of λo/10 (i.e. 1/10 of the wavelength). It has been shown that the performance of the SIW horn can be improved by adding a transition made of parallel plates 116,118 and 120,122 to the substrate 102 beyond the aperture 112, as illustrated in FIG. 1. The advantage of using such a SIW horn antenna is the ability to form the wanted polarization along the z-direction while conforming to a flat brick-oriented solution which at the same time is broadband. This flat-brick-oriented solution is described, e.g., by Marc Esquius-Morote, Benjamin Fuchs, Jean-François Zürcher, and Juan R. Mosig in “Novel Thin and Compact H-Plane SIW Horn Antenna”, IEEE Transactions on Antennas and Propagation, vol. 61, no. 6, pp. 2911-2920, June 2013.
However, a drawback related to SIW horn antennas, including such SIW antennas described above, is the considerable high mutual coupling between neighboring elements when they are placed side by side in an array. In order to avoid visible grating lobes from an antenna array, i.e. unwanted beams which will radiate in other directions, the distance between each antenna element must be equal or smaller than half a wavelength. This limitation in space, plus the need for a compact design, forces the antenna elements to be adjacent to each other producing large mutual coupling.